124 



NORTH AMERICAN INDEX FOSSILS. 



pores open to the surface and form internally a loose vesicular 

 tissue. Ordovicic. 



27. B. laxata Ulrich. (Fig. 182,^.) Ordovicic. 



Massy, originally attached, with basal epitheca ; subovate aper- 

 tures with broad, sharply elevated lunarium ; numerous meso- 

 pores which at intervals form clusters (in weathered specimens) 

 from which zocecia radiate. Interior irregularly and loosely ar- 

 ranged ; walls of granular structure. 



Stones River to Trenton of Minnesota, Iowa and Illinois. 



XXI. Fistulipora McCoy. 

 Form various, the under surface with wrinkled epitheca; cylin- 

 drical or somewhat compressed zocecial tubes with few diaphragms, 





Fig. 183. Fistulipora torta ; a, epithecal side, X I '■> b, celluliferous side, X l '■> c , 

 enlargement of part of b. ( After Hall. ) 



